First and foremost, your job is to keep people alive. Your host of oGCD abilities (Asylum, Assize, Tetragrammaton, etc) should be your first priority for use, Then Afflatus (Solace, Rapture) as they allow you to cast Misery at level 74. If that isn't enough then Regen, Cure2 (single-target), Medica2 and Cure 3 (AoE) should be your go-to spells (as they are your stronger spells), Cure 1 and Medica 1 are your weaker spells for worst-case scenarios (Cure 1 is cheap on mana, Medica1 has more range than Cure3)

As you get more comfortable in the role, you'll find that you don't need to constantly cast healing spells and you can start casting damaging spells to help out with killing things faster.

Jump-potioning is bad because it stops you from organically learning the role. In your case, I'd suggest either picking up AST/SCH to learn to heal properly, or...
Doing every dungeon (besides 50/60/70) in level order so that you can get a kind-of organic leveling experience.